Synthesis of Micron-Sized WS2 Crystallites Using Atomic Layer Deposition and Sulfur Annealing

Abstract

The synthesis of micron-sized WS2 crystallites via atomic layer deposition (ALD) is reported for the first time using bis(t-butylimido)bis(trimethylsilylmethyl)tungsten and H2S as reactants, followed by post-deposition annealing. Self-limiting growth on silicon is demonstrated between 315 and 350 °C. As-deposited films are nanocrystalline, comprising a mix of WS2 and WO3 phases along with residual amounts of WOxNy and SiOx impurities. Post-deposition annealing in elemental sulfur at 600 °C induces the emergence of the 2LA(M), E2g1(Γ), and A1g(Γ) Raman vibration modes for WS2 and increases crystallite size up to a few microns, the largest reported to date for ALD WS2. The growth substrate was found to impact WS2 morphology. Films grown and annealed on silicon produced two distinct morphologies of WS2 crystallites with (i) in-plane multilayered flake pyramids predominantly seen at 650 °C and (ii) out-of-plane “flowers” growing at 700 and 800 °C. Films grown and annealed on ZnS showed only out-of-plane “crumpled” flowers with a high density of WS2 edge sites suitable for catalysis, while films grown and annealed on Al2O3 showed predominately few-layered in-plane WS2 flakes, suitable for electronic devices.